Traditionally, the conductor or conductors in electric cables are made of copper, which may be subjected to various unfavourable influences depending on practical conditions during handling, installation and operation of the cables. Copper conductor cables may in particular be subjected to damage when exposed to elongation above a critical limit. For example, multifunction subsea cables, sea cables and the like are often incorporating signal and/or power cables stranded together with load carrying elements such as steel tubes. Friction forces between the load carrying elements and the copper conductors in signal or power cables cause elongation of the copper in such cables. Thus, when the electric cable is exposed to heavy loads, such as when being deployed at deep sea, the copper conductors may break due to the elongation of the load carrying elements running beside the signal/power cable components. Moreover, an insulation system or insulating layers outside the conductors will be exposed to radial loads, which may lead to so much deformation that insulation failure may be caused.
In connection with problems of the above type, it is well known to armour copper cables with high elongation modulus elements, such as steel wires or composites of synthetic fibres. Such elements prevent the copper conductor material from elongating above the critical limit when exposed to heavy loads, by causing the cable to become a load carrying element itself. This known solution will, however, usually require an unacceptable amount of armouring of the cables in order to withstand the elongation caused by large and heavy cable components, such as steel tubes in specific types of subsea cables, for example the so-called umbilicals.
In addition to the more or less traditional armouring method discussed above, there are other proposals being of some interest in connection with the present invention:
Thus, Norwegian patent application No. 20050753 relates to an electric signal cable comprising at least two insulating conductors, each of the conductors being arranged in a groove in an elongate, central element consisting of an elastic material that makes it possible for the insulated conductors to move in a radial direction when the cable is subjected to longitudinal tension loads.
U.S. Pat. No. 6,424,768 relates to electric cables, in particular ocean bottom cables, with electrical conductors or optical fibres arranged in so-called quads being helically wound around a core. However, this cable structure does not make possible any radially inwards displacement of the conductors when subjected to tension.
International patent publication WO 9214175 describes the incorporation of optical fibres in overhead transmission line groundwires. Although this is quite a remote field of technology from what is of interest to the present invention, there may be a somewhat similar problem: To avoid tensional stress in the fibres. Pliable material in a jacket protects the optical fibres from axial tension by radial flexing of the fibres. A spring-like twisting/untwisting effect is described.